Who do we write for?

Read an interesting article in GQ this morning that discussed the change in Hollywood movies and why producers stick with proven properties rather than risk something new, even though so risks really pay off (The Town, Inception).
So, they said of the main demographics (over and under 25 years of age, males and females). The main demographic was males under 25 years of age because they are the ones going to the movies reliably. Ok...that makes sense. I get the business side, but as an artist, do you continue making movies for the audience you have? If you want a new audience, don't you have to make a movie that they don't even know they want?
Hard to say.
It's the same with writing. Do you write what you think people want to read? Do you write what you like and hope there are others who like the same thing? Obviously you can't please everyone. And there are lots of markets. And art is great because there is room for everyone. Just because Terry Brooks writes great books doesn't mean that Terry Goodkind can't have his share of the market as well.
I am guessing you write what the inner voice tells you to. Maybe it sells and maybe it doesn't bu producing what is in your heart and soul is what makes it art. Producing what you think people want to consume seems more like marketing.

*Note - I'm not taking a stand against marketing or marketers, but I think for me, the order will be first produce art...then market.